A superconducting magnet device is configured by a superconducting coil and a permanent current switch that is arranged in parallel with the superconducting coil. This superconducting electromagnet device supplies current from an excitation power source to the superconducting coil in a state that the permanent current switch is opened, and thereafter reduces the supplied current from the excitation power source to zero in a state that the permanent current switch is closed, so as to realize a permanent current operation in which the current is hardly attenuated but continuously flows through a closed circuit, which is in the superconducting state and configured by the superconducting coil and the permanent current switch. Accordingly, the superconducting electromagnet device can retain a magnetic field over a long period.
When normal conducting transition causes resistance in the superconducting coil during the permanent current operation, energy stored in the superconducting coil is converted to thermal energy by Joule heating, thereby increasing a temperature of the coil. There is a case where consumption of the entire stored energy in the superconducting coil leads to an excessive temperature increase, which in turns causes deterioration in performance of as well as burnout of the superconducting coil. In order to avoid this problem, in the circuit described above, the current is supplied to a protective resistor that is provided in parallel with the superconducting coil after occurrence of the normal conducting transition, and the energy is consumed in the superconducting coil and the protective resistor to suppress the temperature increase of the superconducting coil.
In order to retain constituents represented by the superconducting coil and the permanent current switch in superconducting states, an immersion-cooling method in which the constituents are immersed in a refrigerant represented by liquefied helium or liquefied nitrogen for use, and a conduction-cooling method in which a refrigerator and the constituents are thermally connected by a metal having high thermal conductivity for cooling are widely adopted for conventional superconducting electromagnet devices. However, as for the cooling methods just as described, if a device is enlarged, a large amount of the refrigerant is required in the immersion-cooling method, and a temperature gradient is increased in an object to be cooled in the conduction-cooling method, so that it becomes impossible to retain the object to be cooled at a desired temperature. In view of the above, a forced cooling method in which a refrigerant flowpath is provided in the device to forcedly circulate the refrigerant by a pump is adopted for a large device represented by a nuclear fusion device (PTL 1). Meanwhile, for a medium-sized device represented by a magnetic resonance imaging device (MRI), a thermo-siphon method (PTL 2) is suggested in which the refrigerant circulates in the flowpath by utilizing a density difference between the refrigerant that is vaporized by a heat source such as the superconducting coil and the liquefied refrigerant as well as by natural convection.
In order to retain the superconducting electromagnet device in the superconducting state, the immersion-cooling method, in which a superconducting element is immersed in the refrigerant represented by liquid helium or liquefied nitrogen, and the conduction-cooling method, in which the refrigerator and the constituents are thermally connected by the metal having the high thermal conductivity for cooling, have been widely adopted. However, for the large devices such as the nuclear fusion device and the magnetic resonance imaging device (MRI), the forced cooling method or the thermo-siphon method is adopted in which the refrigerant circulates in the flowpath provided in the device, so as to avoid an increase in an amount of the refrigerant to be used and an increase in the temperature gradient in the object to be cooled.
Here, a process of cooling the superconducting electromagnet device from a normal temperature is referred to as initial cooling. For the initial cooling, in the immersion-cooling method, the refrigerant represented by liquefied nitrogen is first fed into the device from a refrigerant inlet, and the refrigerant that is vaporized by heat in the device is discharged from a refrigerant outlet. Then, since a temperature inside the device reaches a liquefaction temperature of the refrigerant, the refrigerant remains therein while maintaining its liquefied state. The initial cooling is terminated when the superconducting element is immersed.